In the manufacture of semiconductor wafers, microlithography is used to pattern various layers on a wafer. A layer of resist is deposited on the wafer and exposed using an exposure tool and a template such as a mask or reticle. During the exposure process a form of radiant energy such as ultraviolet light is directed through the template to selectively expose the resist in a desired pattern. The resist is then developed to remove either the exposed portions for a positive resist or the unexposed portions for a negative resist, thereby forming a resist mask on the wafer. The resist mask can then be used to protect underlying areas of the wafer during subsequent fabrication processes, such as deposition, etching, or ion implantation processes.
Manufacturers in the field of integrated circuits (ICs) have been trying to reduce the geometric size of the devices (e.g., transistors or polygates) present on integrated circuits. The benefits achieved by reducing device dimensions include higher performance and smaller packaging sizes. Improving lithographic techniques provides improved resolution and results in a potential reduction of the minimum dimension. However, at small geometries, diffraction effects such as proximity effects, poor subject contrast, and poor resolution result, producing wafers with incomplete or erroneous circuit patterns.
A lithographic technique useful at small geometries is known as phase shifting lithography. In phase shifting lithography, the interference between waves of an exposure energy is used to overcome diffraction effects and to improve the resolution and depth of optical images projected onto a target. Phase shifting lithography involves controlling the phase of an exposure light at the target such that adjacent bright areas are formed preferably 180 degrees out of phase with one another. Dark regions are thus produced between the bright areas by destructive interference even when diffraction would otherwise cause these areas to be lit. This technique improves total resolution at the target (i.e., wafer) and allows resolutions as fine as 0.10 microns to occur.
In the past, phase shifting templates have been used experimentally to print submicron features. Phase shifting lithography is still in the research and development stage, however, and has not been used extensively for commercial volume semiconductor manufacture. One reason phase shifting lithography is not widely used commercially is the high defect density which results during its manufacture. Phase shifting templates are difficult to form without defects and any defects on the template may be printed onto the target. In addition, an individual reticle costs in the range of $10,000 to $20,000 and typically requires up to two weeks to manufacture. Mask production likewise involves substantial time and expense. The complete circuit patterning for a modern IC will typically require 10 to 20 or more reticles.
The most common template defects are pattern distortions of two types: opaque defects and clear defects. Reticles and masks typically consist of an opaque thin film of metal or metal silicide, such as chromium or molybdenum silicide, deposited in a pattern on a transparent substrate of quartz, glass, or fused silica. Opaque defects, which may occur as spots, pattern extensions, bridges between adjacent patterns, or the like, are the result of opaque material such as chromium or molybdenum silicide being present in a non-pattern area. Clear defects, which generally occur as pinholes, missing parts, or breaks in the pattern, are the result of missing or inadequate layers of opaque material in a pattern area on the template.
Focused ion beams (FIBs) have been used for repair of optical masks and reticles since the mid-1980s. The ability of the FIB to accurately remove unwanted portions of the metal film and to deposit material to "edit" the pattern makes it potentially an almost ideal repair tool. A FIB exposes a template to a beam of positively charged ions, typically gallium ions, via an optic system. When a template is exposed to the ion beam, secondary ions are produced, and may be detected by the FIB machine and monitored to determine the progress of repair work. If a chromium pattern is exposed, secondary chromium ions are generated, and if a silicon, glass, or molybdenum silicide pattern is exposed, secondary silicon ions are generated.
Sputtering with a scanning FIB is the preferred method of opaque defect repair at small geometries, but FIB sputtering has several disadvantages. First, difficulty in precisely determining the endpoint when etching molybdenum silicide films leads to overetching and subsequent recess formation in the template substrate, which affects the phase shifting amount and may cause transmission error. Second, the high energy (25 to 50 KeV) FIB beams used cause significant template damage during repair due to the beam's high sputter rate. In addition, significant amounts of ions from the ion beam are implanted into the template substrate during imaging and opaque defect repair, resulting in an effect called "ion staining" or "gallium staining", when a gallium ion beam is used. This effect causes local reductions of the substrate's transparency which may print on the semiconductor wafer, and/or may be identified erroneously as defects by industry-standard mask inspection equipment.
There is needed, therefore, a method of repairing opaque defects on a phase shifting template that reduces or prevents overetching, and that reduces damage caused by ion staining.